This invention relates to a plastic mold type semiconductor device, and more particularly to the connecting structure between the leads and the semiconductor elements.
Sealing or molding in semiconductor devices such as ICs, LSIs, or the like, protects semiconductor chips from pollution sources such as dust, chemicals, gas, moisture, and the like, and avoid mechanical breakages of these elements. As a protective method, hermetic sealing and plastic molding are generally used. Resin or plastic material used in this sealing or molding closely contacts the element surface and completely covers thereover it. Semiconductor devices protected by plastic material or resin are referred to as a plastic mold type semiconductor device.
Semiconductor devices of this kind are known as the DIP (Dual Inline Package) type, in which two rows of lead pins are arranged in parallel: semiconductor devices of the SIL type in which a single row of lead pins is linearly arranged, and the like. A conventional plastic mold type semiconductor device will be described by taking an example of a DIP type semiconductor device.
A semiconductor chip mounted on an element mounting portion at the central portion of a lead frame having a flat smooth surface and front end portions of a plurality of leads arranged so as to surround the element mounting portion are connected by means of bonding wires. Thereafter, by transferring plastic material into a cavity or hollowed portion with the lead frame being held between the upper and lower molds in order that the semiconductor chip is positioned, e.g., at the central portion of the cavity formed on the contact surface of the upper and lower molds to fill the cavity with plastic material, the semiconductor chip mounted on the element mounting portion, bonding wires and inner end portions of leads, etc. are embedded into a plastic mold in a predetermined form and sealed. In this case, the following technique is typically employed. At the peripheral portion of the cavity formed on the contact surface of the upper and lower molds, plastic material filled in the gaps between a plurality of adjacent leads is intercepted by a dam bar surrounding the outer peripheral portion of the cavity at a predetermined interval to connect the plurality of leads. The semiconductor chip is bonded onto the chip mounting portion (bed) by a conductive adhesive agent such as solder, gold, plastic material, etc. After the plastic material is filled in, the lead frame portion located outside of the molded resin is reformed. That is, any unnecessary portion, including the dam bar, is cut off. Thus, a plastic mold type semiconductor device of the DIP type is provided.
In recent years, with high density integration of semiconductor elements, semiconductor chips tend to large in size. On the contrary, not only plastic mold type semiconductor devices, but also semiconductor devices of other types, need to be miniaturized similarly to other electronic parts. Accordingly, the package size is maintained at an existing size, or must be as small as possible. As a result, the size of the package becomes extremely close to that of the semiconductor chip, so the inner lead portion of the lead (which indicates the front end portion close to the element of the lead, and is ordinarily sealed with a molded plastic material) cannot take a sufficient molding length within the molded plastic material. For this reason, since the strength cannot be increased, troubles may occur, such that leads are drawn or pulled out during the process succeeding formation of mold.
To reduce occurrences of such troubles, an arrangement is proposed to elongate the inner lead up to the portion above the semiconductor chip. When the lead is so arranged, the portion coated with the molded plastic material is elongated.
However, even in employing this structure, since the inner lead portion is positioned above the chip, it may be easily moved toward the surface of the chip, even if only a small force is applied thereto. During the wire bonding process, the inner lead portion of the lead is pressed toward the semiconductor chip at the time of bonding. At this time, if there is any foreign substance such as dust on the semiconductor chip, such foreign material will be put between the semiconductor chip and the inner lead portion, with the result that the semiconductor is scratched or damaged due to the pressure on the upper surface of the inner lead at the wire bonding. Where the substrate of the semiconductor chip is silicon, a silicon particle is assumed present.